Height and density of clouds are unheeded. Thermometer readings, on the other hand, are more definite, and less subject to erroneous classification; and we would rather accept their testimony of lunar influence on terrestrial temperatures, indirect though it be, than anything not susceptible of instrumental measurement. It would be a great point if we could measure the moon's dark heat directly, but this there seems little hope of doing, unless, as Professor Smyth suggests, we ascend to the level of the high clouds which he saw evaporated by the full moon-an altitude of about three miles.

The visible heat rays, and those which are not intercepted by the atmosphere, we can now, thanks to modern means of thermometry, measure with some pretensions to accuracy. Whether the moon's light possesses perceptible warmth has been a point of anxious and tentative inquiry among astronomers and physicists for a century and a half at least. The first observer who recorded a trial of it was, I believe, Geminiano Montanari, who, in 1685, thought he had indications of lunar heat in an ordinary dilatation thermometer. The second was Tschirnausen, the famous burning-glass maker. He published, in the year 1699, an account of a wonderful double lens, which concentrated the solar rays so far as to make them melt and fuse metals. It was formed of a four-foot burning-glass, with another lens of smaller size behind it. In telling of a number of its achievements, he mentions, little more than incidentally, that he turned it upon the full moon, but, though the image formed in the focus was of great brilliancy, there was no sensible heat. He does not say what thermometer he used. Six years after, in 1705, La Hire the younger made an experiment with a burning mirror, belonging to the Paris Observatory, of 35 inches diameter, and an air and mercury thermometer of the construction then recently proposed by Amontons. The bulb of this instrument was two inches in diameter, and when it was placed at the focus of the mirror the moon's reflected image just covered it. The height of the mercury was noted, and the condensed moonlight was kept upon the bulb for a considerable time, but there was no alteration in the reading. This one trial satisfied La Hire that the moon's light was heatless.†

We find no record of attempts at lunar thermometry during the hundred years following the date of that of La Hire. But in 1820 attention was again turned to the subject by Professor Howard, of the Maryland University in the United States. Thermometers had by this time improved in construction: the instrument used by Howard was a modification of the differential one proposed by Leslie. The condenser was a mirror thirteen

"Histoire de l'Académie," 1699, p. 90.

† Ibid., 1705, p. 346.

inches in diameter, and one bulb of the thermometer, previously blackened to increase its heat-absorbing power, was mounted at the focus. The reflector was opposed to the light of the full moon, and, to quote the observer's words, "the liquid began immediately to sink, and in half a minute was depressed eight degrees, when it became stationary. On placing a screen between the mirror and the moon, it rose again to the first level, and was again depressed on removing this obstacle. I repeated this experiment several times to satisfy myself, and some of my friends who happened to be present, that there was no fallacy in the conclusion of its being a positive proof of the calorific power of the lunar rays, and at the same time affording an evidence of the great delicacy of the instrument."* The eight degrees here indicated were not those of any recognised thermometric scale, but mere arbitrary divisions upon the tube, about a millimètre apart. Within a year or two Pictet repeated Howard's experiment, using a similar thermoscope, but his index remained unmoved under lunar influence; if it altered at all, it gave an indication of cold. Prevost, reporting these results, pointed out that a mirror apparently reflects cold when exposed to the clear sky, because it intercepts the earth's warmth, and leaves the thermometer free to radiate its heat towards the sky. He thought the same effects would be produced whether the reflector be turned to the moon or to any other part of the heavens; and he suggested the desirability of experimenters making this test a part of their lunar thermoscopic observations. Further, he remarked that heat might be experienced occasionally, on fine summer nights for instance, for then the upper air is warmer than the lower. Howard's strongly manifested warmth may have come from this source; possibly Montanari's also, and some detected by Frisius in 1781. Professor Volpicelli, in some historical notes on lunar thermometry, mentions certain researches by a Mr. Watt, which seemed to confirm Howard's experience. This was Mr. Mark Watt, a member of the Wernerian Society. His experiments, besides being rather unphilosophical, hardly bear upon the point: they refer rather to some supposed attractive and repulsive actions exercised by the moon's light upon little discs of metal mounted at the ends of a balanced bar.‡ There is mention of heat from the lunar rays, but it is ambiguous.

The experiments so far prosecuted, while they evidenced the anxiety of physicists to settle the point at issue, only proved the inefficacy of the means which, in expansion thermometers, they possessed for the purpose. But presently came a revolu"Silliman's Journal," vol. ii. p. 329.

"Bibliothèque universelle," xix. 35.

"Edinburgh New Philosophical Journal," xix. 122.

tion in the science of heat measurement: Seebeck, in 1822, discovered the power of thermal currents to excite electricity in metallic conductors; and Nobili adapted the discovery to the construction of a thermoscope which permitted the making of experiments that would have been declared impossible a very short time before. The years about 1830 found the inventor of the thermopile, in conjunction with Melloni, engaged upon investigations involving the measurement of temperatures separated by very small fractions of a degree upon any existing scale. The passage of heat through transparent bodies, the bodily warmth of insects, the calorific exhalations accompanying the luminous glow of phosphorus-these were the subjects upon which were first tested the powers of the new instrument, which was so delicate that it felt the warmth of a human body thirty feet removed from it. Having refuted the common opinion that luminous phosphorus exhibited the phenomenon of light without heat, Melloni sought to disprove the similar idea sometimes entertained with regard to the light of the moon. The pile he employed was formed of thirty-eight pairs of bismuth and antimony bars, soldered together at their alternate ends, and packed closely, though electrically isolated, into a metallic hoop, the first and last bars being connected by wires with the coil of a delicate galvanometer. A conical reflector surrounded the exposed ends of the bundle of bars which formed the "face" of the pile; this face was coated with lamp-black: the lunar rays were concentrated upon it by a concave metallic mirror, the diameter of which Melloni does not mention; but whatever effect they might have produced was completely shrouded by that of the cold received from the sky, or, to speak more correctly, the heat escaping by radiation from the exposed surface of the pile.* It must be borne in mind that currents are developed in the bundle of plates or bars whenever the equilibrium of temperature between the two faces is disturbed in any way. If one face is boxed up and the other is opened to the view of a clear sky, the latter will cool more rapidly than the former, and the effect will be as if the covered face had been actually warmed. Melloni, in his lunar experiments, found his galvanometer always pushed to its limit of divergence from this cause; and for a time he was frustrated. Several years after, adopting precautions to neutralise the frigorific effect of the clear sky, he was able to obtain decided heat indications, as we shall presently see. In the meantime, Professor Forbes, taking advantage of the delicacy and prompt action of a pile he had constructed for some experiments upon the refraction and polarisation of heat, renewed Melloni's experiment, using a lens

"Annales de Chimie," xlviii. 211.

instead of a mirror, and thereby preventing the escape of the pile's heat towards the sky. The lens in question was of the polyzonal form, having a diameter of thirty inches and a focal length of forty-one inches; giving an image of the full moon 0.38 inch in diameter, and thus securing a concentration of its light and heat to the extent of 6,000 times. A high full moon in December 1834 furnished good conditions for a trial: the pile was mounted in the focus of the lens, and the moon's rays were alternately thrown upon and screened from it some twenty times during the hour and a quarter that the experiments lasted. There was an occasional movement of the needle amounting to not more than a quarter of a degree, but, as Forbes pointed out, the greater part of this was due to the dynamical effect of an instantaneous impulse. To form an estimate of the amount of heat which this deflection represented, the number of degrees upon the galvanometer scale equivalent to a centigrade degree was ascertained by exposing the pile and a thermometer to one and the same source of heat. Two thermometers were, in fact, used for this comparison; and the mean value of a degree's deflection of the needle was found to be onefiftieth of a centigrade degree. Supposing that the statical effect of the moon-heat upon the pile-needle amounted to oneeighth of a degree, we have evidence of heat to the extent of

th of a degree centigrade. But this was the measure of the condensed heat: assuming that the lens, in consequence of dispersion, reflection, and absorption, concentrated the light 3,000 instead of 6,000 times, and making a further allowance for the proportion of the pile-face covered by the moon's image, Forbes concluded it as improbable that the direct light of the moon would raise a thermometer one-three-hundred-thousandth part, or, decimally written, 0-0000033 of a centigrade degree.* Judging from later results, it seems probable that the galvanometer needle employed by Forbes was not sufficiently delicate to answer to the action of the current furnished by the pile.

Melloni's new experiments were made in 1846 with a polyzonal lens of a mètre diameter, constructed for the Meteorological Observatory at Vesuvius. At first he was plagued with the abstraction of heat from his pile, as he had been before when using a reflector; but he got over his difficulty by mounting the lens and pile within doors, allowing the moonlight to enter through an open window. When all was in readiness for observation, and the rays were first cast upon the instrument, the needle moved several degrees in the direction indicating heat; but upon covering and re-exposing the face of the pile,

* "Transactions of Edinburgh Roy. Soc.," xiii. 139.

much to Melloni's surprise, it turned towards cold. He traced the cause of this to draughts of cool air impinging upon the thermoscope. These could have been remedied by ordinary means; but the more philosophical cure was to cover the pile with plates of glass, which would allow heat to pass through to it but yet keep all air-tight. Two screens were then interposed, and upon the next favourable occasion the experiments were renewed. This time they were perfectly successful: the needle, stationary when the pile was uncovered, soon began to move slowly to the heat-side of the scale, and at the end of five minutes' exposure it reached its maximum of deflection, 3.7 degrees of arc. The pile was covered, and it returned to zero; uncovered, and it turned heat-wise. This alternation was repeated many times, and always with the same qualitative result. Subsequently, trials were made in the presence of MM. Belli, Mossotti, and Lavagna, and many other distinguished savans, all of whom went from the chamber, says Melloni, convinced that the light of the moon is calorific.* No detailed observations are given by Melloni, but he says that he assured himself that the lunar heat varies, as one might suppose it would, with the age of the moon and with its height above the horizon: he intended to determine the actual amount of the heating effect, but it does not appear that he ever did so. Prof. Zantedeschi confirmed Melloni's conclusions in 1848 by observations made with a mirror of 0.6 mètre diameter, and a pile made by Gourjon of Paris, used upon the clear full moons that shone in the summer sky of Venice.

Favourable circumstances offered for catching some of the heat-rays that are ordinarily absorbed by the lower atmosphere, when Professor Piazzi Smyth, in 1856, ascended Teneriffe to test the supposed advantages of an elevated station for delicate astronomical observation. A thermopile accordingly formed part of the equipment of the expedition, but no burning lens or mirror was taken to be used with it. The pile was simply furnished with a polished metal cone of rather larger base than the area of the face, and the experiments were made by alternately turning the cone towards the moon and to a part of the sky 20 degrees east or west of it. Two nights in August 1856, that of the full moon and the following one, afforded favourable opportunities for observation, though the luminary was rather low, having an altitude of about 45 degrees. The effects of heat were unmistakable: on each night some thirty readings of the galvanometer were taken with the pile alternately on and off the moon; and the mean deflection in the direction indicating warmth derived from all the

"Comptes rendus," xxii. 543.